This invention relates generally to the field of heat treatment of metals, and more specifically to a method and system of providing real-time, closed-loop control of an induction heating machine by using an acoustic sensor to measure, in real time, changes in characteristic material properties of a workpiece during the process of induction heating. The method includes the steps of generating an acoustic (e.g. ultrasonic) wave in the workpiece with a pulsed laser; optically measuring displacements of the surface of the workpiece in response to the acoustic wave; and calculating the sub-surface material properties by analyzing the measured displacements.
Induction heating is a well-known process for efficiently applying energy directly to metals and other conductive materials for heat treating, melting, welding, brazing, tempering, normalizing, aging, or pre-heating prior to hot working. Induction heating can also be used in non-metal applications, including adhesive bonding, graphitizing carbon, drying, curing, and superheating glass. Some parts, such as those in turbine engines, have extremely different material requirements in different regions of the part. Heat processing of such parts requires non-uniform thermal distribution that must be precisely controlled. In the induction heating process, alternating electric current is passed through an induction heating coil that is positioned closely to a workpiece. Where the lines of magnetic flux produced by the induction heating coil enter the workpiece, the alternating magnetic fields induce an alternating electric potential (e.g. voltage) in the workpiece. The alternating electric potential drives eddy currents in a thin surface layer. These eddy currents dissipate energy within the surface layer by resistive Joule heating losses. The depth of resistive heating (e.g. skin depth) is inversely proportional to the square root of the product of three parameters: applied induction frequency, magnetic permeability, and electrical conductivity. The resultant steep temperature rise in the resistively heated surface layer is related to the specific heat, density, thermal conductivity, power level, and duration of heating. Magnetic coupling of the induction heating coil to the workpiece depends strongly on the geometrical arrangement, among other properties.
A common use of induction heating is case hardening of medium-carbon steel parts, such as gears, axles, and driveshafts. Many industrial applications require a steel part having a hardened outer surface (e.g. xe2x80x9ccasexe2x80x9d) and an interior region of higher toughness to provide improved strength, wear resistance, fatigue life, and toughness. Other applications include induction hardening of crankshafts, valve seats, railroad rails, rolling-mill rolls, and hand tools. Induction heating rapidly heats the outer surface layer of the steel workpiece in a short period of time (e.g. 5 seconds). This produces a very large temperature gradient through the depth, which can be as large as 100 C/mm. Above a critical transition temperature (about 760 C for 1050M steel with 0.45% C) the initial ferrite-pearlite microstructure (BCC) transforms into the austenite phase (FCC). The depth of the xe2x80x9chot boundary layerxe2x80x9d is commonly 1-5 mm, depending on the heating time and material properties, for induction heated steel shafts.
Upon continued heating of the part, the transformed austenite layer thickens and extends deeper from the surface. Optimum peak surface temperatures can be 870-925 C, depending on the carbon concentration, and the desired depth of hardening. For some applications, the peak surface temperature can be as high as 1200 C. Final hardening of the outer layer occurs when the heating power is shut off and the part is quenched (e.g. rapidly cooled from the outside to less than 200-400 C in 10-20 seconds). This converts the austenitic layer into a hard, metastable martensitic phase with a Rockwell hardness of Rc=50-60. An optional tempering step can follow the quench cycle, which can further improve the metallurgical properties.
Induction hardened steel parts are designed to have a case hardened layer with a specific desired depth. For example, a 25 mm diameter 1050M steel automobile axle may be designed with a hardened layer from 4-5 mm thick, as defined by a Rockwell hardness of at least Rc=50. Should the layer be too thin, the axle would wear too quickly or have insufficient strength; should the layer be too thick, the axle would be too brittle. During mass production, the measured case depth should be repeatable to within +/xe2x88x920.1 mm. This requires close control of the induction heating process, as well as tight control of material properties, chemistry, workpiece alignment, etc.
Closed-loop control of the induction heating and hardening processes has been an elusive goal of the industry for many years. Existing induction hardening equipment is typically operated with open-loop process controllers, wherein an operator manually selects power and time (e.g. heating duration). Production users of this equipment monitor the process by destructively sectioning finished parts and inspecting the results; i.e., a finished part is cut apart and the case depth is directly measured radially across the cross-section by using a Rockwell hardness indentor, metallographic inspection, or chemical analysis of the carbon concentration profile. Process development for new parts is accomplished by time-consuming and expensive trial-and-error; for a given coil and part design, heating and quenching parameters are varied until destructive analysis reveals that the desired hardness profile is being produced.
These parameters are then utilized in the production run and the hardened parts are sampled and analyzed at regular intervals for quality control and assurance. If the tested part is bad, the production run from the previously tested good part is sampled to determine where the process failed. Production equipment may be taken out of service until subsequent parts test satisfactory. Since each test can take a minimum of several minutes by a trained technician, this process is quite inefficient for mass production. Unfortunately, small variations in the steel""s chemistry and microstructure can produce unacceptably large variations in the measured case depths, even for nominally acceptable material specifications. The cause of these variations is not well understood.
Other sources of variability include improper part positioning (e.g. misalignment relative to the heating coil), defects in the part (e.g. cracks), and damaged or aged heating coils. Low hardness values measured on a finished part may be caused by: surface decarburization; lower carbon content than specified; inadequate austenitizing temperature; prior structure; retained austenite (mostly in high-carbon steels); and unsatisfactory quenching.
What is needed is a real-time, non-destructive, non-contact diagnostic technique that can respond quickly to the temperature changes and phase transformations in the workpiece during the induction heating process. The diagnostic should be small enough to provide sufficient spatial resolution, and robust enough to withstand the hostile environment (e.g. high temperatures, high magnetic fields, rotating parts, and large volumes of quenching fluids). Use of an active feedback of process information measured directly from the part, coupled with closed-loop control of the heating process, would greatly improve the efficiency of induction hardening systems, while increasing accuracy and reducing part rework.
Direct measurement of the workpiece""s surface temperature during induction heating could provide a useful signal for closed-loop feedback control. However, use of contact thermocouples is impractical for mass production, especially since cylindrical parts are often rotated at significant rpm""s to create uniform heating profiles. Non-contact optical pyrometry could be used, however the accuracy is affected by surface conditions (e.g. emissivity) and the operating environment (e.g. smoke, dust, vapors). Coating of the pyrometer""s window by the quenching fluid can also degrade accuracy. Historically, pyrometers have not had a sufficiently fast response time to monitor the rapid changes in surface temperature during induction heating. Neither pyrometry, nor surface-attached thermocouples, can directly measure the internal temperatures within a workpiece.
Indirect measurement of the workpiece""s temperature, and/or temperature profile through the depth, can be inferred by measuring corresponding changes in the elastic, metallurgical, electrical, and magnetic properties of the workpiece as it heats up during induction heating. For example, the resistivity of medium-carbon steels can increase as much as 800% as the temperature increases from 20 C to 900 C.
The average electrical resistance of the workpiece (e.g. averaged over the cross-sectional area) can be measured indirectly by monitoring the voltage, current, and phase of the induction heating coil. This approach is described in U.S. Pat. No. 5,630,957 (commonly assigned to Sandia Corporation), which is herein incorporated by reference. In this patent, Adkins et al. teach a method of closed-loop control of an induction hardening machine that uses a trained neural network processor, combined with real-time measurement of the voltage, current, and phase in the induction coil, as measured by a Rogowski coil surrounding a current lead. The depth of hardening is controlled, in part, by computing the energy absorbed by the workpiece, and the changes in the average resistance of the coil plus the workpiece during the heating duration. However, this method does not provide any direct information regarding the temperature profile through the depth, or local information at a specific point on the workpiece.
Acoustic sensors can provide local, non-destructive, non-contact diagnostic information about variations in the material properties throughout the depth. Acoustic sensors rely on the fact that the speed of sound for longitudinal elastic (e.g. acoustic) waves is proportional to the square root of the ratio of the elastic modulus divided by the density. For other wave modes, such as Rayleigh surface waves, Lamb waves, etc., the relationship between wave speed and the physical properties have more complicated equations. Any variation in these elastic properties and microstructure, or the presence of internal boundaries, voids, sharp gradients, etc. manifest themselves in elastic wave time-of-flight (TOF) variations, internal reflections, and acoustic interference patterns that can be measured and/or imaged by optical methods looking at the surface of the part.
Acoustic sensors have been used to indirectly measure the depth of case hardening on finished parts. U.S. Pat. No. 5,648,611 to Singh et al. discloses a method of measuring the case depth by (1) launching an acoustic wave along the surface of the specimen such that the wave passes through the case (e.g. hardened layer), (2) determining the velocity of the wave by time-of-flight (TOF) measurement; and (3) comparing the measured velocity with a correlation previously established between wave velocity and case depth for the same wave frequency. This patent teaches the use of one or more electromagnetic acoustic transducers (EMAT""s) to generate and detect the ultrasonic pulses. A serious problem with using EMAT""s for real-time control of the induction heating process is the requirement for EMAT""s to be in close proximity (e.g.  less than 1 mm) to the specimen""s surface. Typically, EMAT""s do not require coupling fluids, but piezoelectric transducers do. For high temperature application, water cooling is necessary to keep the magnet well below its Curie transition temperature if a permanent magnet is used. For inductively heated parts, where the surface temperatures can exceed 900 C, the part is often rotated and subjected to large volumes of quenching fluids. In this case, the use of EMAT""s present serious difficulties in their practical application because of the requirement that they be located in close proximity to the heated surface.
A preferred alternative to using EMAT""s for process control of inductive heating is using laser ultrasonic (LU) sensors to generate and detect acoustic waves. Laser ultrasonic sensors are non-contacting, and can be placed at a much larger distance from the specimen""s surface than EMAT""s. Fiber optic cables can be used to carry the laser beams. Laser ultrasonic techniques have been used to measure the wall thickness of tubes during production. (see U.S. Pat. No. 6,078,397 to Monchalin, et al.). Another common use of laser ultrasonic sensors is measurement of the bulk temperature of a silicon semiconductor wafer during thermal processing (see U.S. Pat. No. 5,724,138 to Reich and Kotidis).
Acoustic sensors have been used to measure anisotropic characteristics of properties, especially those resulting metallurgical treatments such as rolling, forming, extruding, drawings, and forging. Those treatments create anisotropy in the grain size, texture, and crystal orientation, which can be detected with laser ultrasonic techniques (see U.S. Pat. No. 5,804,727 to Min and Lu, which is herein incorporated by reference).
Laser ultrasonic methods have been demonstrated in the laboratory to measure the elastic modulii of various titanium-hydrogen alloys over the range of 20-1100 C. See O. N. Senkov, M. DuBois, and J. J. Jonas. xe2x80x9cElastic Modulii of Titanium-Hydrogen Alloys in the Temperature Range of 20 to 100 C,xe2x80x9d Metallurgical and Materials Transactions A, 27A, Dec., 1996, pp 3963-3969. In that work, acoustic waves were generated using 6 ns long laser pulses, and then detected with a confocal Fabry-Perot laser interferometer to sense the variation in elastic modulii between bars having different, but uniform, hydrogen concentration. In those measurements, the temperature of the bar was uniform and homogeneous.
Laser ultrasonic techniques have been disclosed that can provide real-time industrial process control (see U.S. Pat. No. 5,286,313 to Schultz, et al.). However, Schultz does not discuss the use of laser ultrasonic techniques for real-time control of the induction heating process. As discussed earlier, induction heating typically creates very steep temperature gradients (e.g. 100 C/mm) and metallurgical phase changes in a thin, hot boundary layer (1-5 mm), especially during the induction hardening process. This highly complex, non-linear temperature profile creates unique problems with the interpretation and understanding of data generated by laser ultrasonic sensors.
The very steep temperature profile created by induction heating is to be contrasted with conventional furnace heating, such as used for annealing or heat treating, where the workpiece temperature is essentially uniform and homogenous during heating. In this case of uniform temperature throughout the body, simple corrections can be used for the temperature-dependent elastic wave velocity. Such a simple approach is not suitable for monitoring the steep temperature gradients produced during induction heating, especially for induction hardening processes.
Against this background, the present invention was developed.